Many integrated circuits (“ICs”) and circuit assemblies (“circuit boards”) use fiber-reinforced substrate material. Glass fibers or fibers of other materials are mixed in polymer matrix, such as an epoxy matrix, to form a sheet of fiber-reinforced material that is processed into a substrate for mounting an IC, a package base, a printed circuit board (“PCB”) (also known as a printed wiring board), adaptor, or interposer. For purposes of convenient discussion, such patterned fiber-reinforced polymer components will be referred to as “PCBs”, regardless of eventual application.
Many technologies have been developed for fabricating PCBs, and for fabricating the various features found on a PCB, such as traces, vias, and through-holes. The techniques suitable for producing a PCB for a particular application depend on various performance requirements, such as pitch. Pitch is basically how close together conductive vias (generally, solid conductive structures extending from one surface of the substrate to the opposite surface of the substrate) or plated through-holes (“PTHs”, which are generally similar to conductive vias except that an outer wall of the plated through-hole is conductive and a hole extends from one surface of the substrate to the opposite surface). For PCBs having a coarse pitch, techniques such as mechanical drilling and solder-paste screening are acceptable. For PCBs having a fine (narrow) pitch, other techniques, such as laser drilling and electroless copper (“e-Cu”) plating techniques are used. Electroless copper plating can be used to plate non-conductive surfaces, such as the substrate material of a PCB; however, electroless copper plating uses strong ionic solutions.
Conductive anodic filaments (“CAFs”) occur when ions, particularly residual ions from the electroless plating process, travel along a fiber to form a conductive path. Generally, the fiber or fiber-polymer interface provides an electrolytic pathway for the transportation of reaction products that occurs in the presence of humidity. A voltage applied between conductive features (i.e., and anode feature and a cathode feature) promotes copper dissolution at the anode and copper migration toward the cathode until a conductive path is formed. CAFs are not a major problem on coarse-pitch PCBs, but increasingly appears on fine-pitch PCBs as internal surfaces of the PCBs (i.e., via walls or plated through-hole walls) are fabricated closer together. CAFs occur when copper ions used in the standard e-Cu plating process travel along a fiber (e.g., a glass fiber) that intersects an internal surface of the fiber-reinforced polymer composite.
CAFs are enhanced by high humidity during storage or use, by high voltage gradient between anode and cathode, by certain soldering flux ingredients, by mechanical hole drilling (which can induce fiber-matrix separation), multiple thermal cycles during processing, and by higher processing temperatures associated with lead-free solders. CAFs are particularly likely to occur if the fiber separates from the polymer matrix, even slightly, to allow copper ion migration and exchange along the length of the fiber. If a conductive path is established from one internal surface (i.e., one conductive via or plated through hole) to another, an electrical short is established). Whether this occurs depends on many factors, such as the distance between adjacent internal surfaces, the integrity of the fiber-polymer interface, and the processing parameters of the e-Cu process, such as plating time (target copper thickness).
Many efforts have been made to reduce CAFs. One approach has focused on reducing the separation between the fiber and polymer matrix. Another approach has been to tighten the drilling process control, since adjacent holes drilled at the edge of the drilling tolerance specification are closer together, and hence more likely to short together due to CAF. Another approach has been to utilize laser drilling, which is less likely to induce fiber-matrix separation than mechanical drilling. Another approach has been to modify the parameters of the e-Cu process to reduce the chance that a fiber will be able to form a short from one end to the other during copper plating. All of these approaches tighten the allowable materials or process window(s) and increase cost. CAFs remain a problem, and in-line monitoring is typically done on a sample basis, allowing some components with CAFs to proceed to the next assembly. Even if the in-line monitoring identifies bad parts, it results in scrapped PCBs.
FIG. 1 is a side view of a portion of a conventional PCB 100 with plated through holes 102, 104 illustrating a CAF 106. The plated through holes 102, 104 extend from a first major surface 108 to a second major surface 110 of the PCB 100, which is a fiber-reinforced polymer composite material, such as glass-fiber reinforced epoxy. Glass fibers 112, 114 are shown in the central portion 116 of the PCB 100, but are not shown in the side portions 118, 120 of the PCB for simplicity and clarity of illustration, although fibers are generally present throughout the PCB composite material.
A layer of copper has been formed using an e-Cu plating process to create conductive cylinders 122, 124 lining the interior surface of the PTHs 102, 104. During the plating process, copper ions migrate along a fiber 114 to form conductive portions (i.e., copper-plated portions) 126, 128 of the fiber. The conductive portions of the fiber can extend from both ends, and when they merge, a CAF 106 is formed that electrically connects (shorts) the first PTH 102 to the second PTH 104.
FIG. 2 is a side view of a portion of a conventional PCB 200 with conductive via (“blind via”) 202 illustrating a CAF 206. The CAF 206 extends from a first blind via 202 through a fiber-matrix portion of the PCB 204 to a second blind via 208, which forms an electrical short between the blind vias 202, 208.
Techniques for fabricating PCBs without CAFs that avoid the limitations of the prior art are desired.